Nitrogen oxide and hydrocyanic acid production method

ABSTRACT

A metal fiber based on one or several elements from the group of platinum, palladium, rhodium, ruthenium, and iridium with 0 to 30% by weight of one or several additional alloy elements from the group of nickel, cobalt, gold, rhenium, molybdenum, and tungsten, contains 1 to 500 ppm by weight of boron or phosphorus. A non-woven material or netting, in particular for the production of nitrogen oxide or for the production of hydrocyanic acid, is made of such fibers. For the production of fibers based on noble metals having up to 30% by weight of additional alloy metals by drawing the fibers from a melt, the melting point of the metal is reduced by at least 400° C., before drawing of the fibers, by additionally alloying with boron or phosphorus, and the boron or the phosphorus is removed again from the fibers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/058,373, which is a Section 371 of International ApplicationNo. PCT/EP2009/005521, filed Jul. 30, 2009, which was published in theGerman language on Feb. 18, 2010, under International Publication No. WO2010/017894 A2 and the disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to catalyst structures for the productionof nitric acid and hydrocyanic acid, to metal fibers based on noblemetals and suitable for the production of catalysts, to netting andnon-wovens made therefrom, and to processes for the production of suchfibers, non-wovens and nettings.

German Patent DE 199 45 742 C1 discloses a metal fiber catalyst body,the metal fibers of which are produced by melt extraction. Using textiletechniques, non-woven fabrics, among others, are produced from thefibers. The catalyst material platinum, palladium rhodium is containedin the fibers or present in the woven material as additional fibers.

German published patent application DE 100 00 097 A1 discloses meltextraction processes for metal fiber production from thin fibers withdiameters of less than 100 μm using melt bath temperatures of up toabove 1500° C.

German published patent application DE 197 12 625 A1 describes a processin which metallic fibers, among others, are deposited through the meshesof a distributing roller onto a moving substrate and joined together atintersection points.

These processes seem to be unsuitable or at least difficult to implementfor the production of metal fibers and nettings or non-wovens producedtherefrom in the case of metals with a high melting point.

German Patent DE 100 40 591 C1 teaches the method of alloying platinum,iridium, rhodium and ruthenium, among others, with boron and phosphorus.

The efficiency of known catalysts, in particular platinum-rhodiumcatalysts, decreases over time.

BRIEF SUMMARY OF THE INVENTION

It is the object of the present invention to produce fibers, netting ornon-wovens based on noble metals. The process should be as simple aspossible. The catalysts should retain a high level of efficiency in thelong term.

To achieve this object, the melting point of a metal is drasticallyreduced by boron or phosphorus before processing the fibers, and theboron or phosphorus is largely removed subsequent to the production offibers or non-wovens or nettings produced from the fibers.

The object is further achieved by a non-woven material or nettingcomprising metal fibers based on one or more elements from the group ofplatinum, palladium, rhodium, ruthenium, and iridium with 0 to 30% byweight of one or more additional alloy elements from the group ofnickel, cobalt, gold, rhenium, molybdenum, and tungsten, wherein themetal fibers contain 1 to 500 ppm by weight of boron or phosphorus.

Still further, the object is achieved by a process for the production offibers based on noble metals with up to 30% by weight of additionalalloy metals by drawing the fibers from a melt, wherein the meltingpoint of the metal is reduced by at least 400° C. before drawing thefibers by additionally alloying with boron or phosphorus, and the boronor the phosphorus is removed again from the fibers, and wherein thefibers are sintered to form a non-woven material or netting.

It is assumed that the surface roughness of the fibers is increasedduring the removal of the boron and/or phosphorus and that theeffectiveness of the catalyst is increased simultaneously with thisadditional porosity. In addition, it is suspected that boron orphosphorus hampers the formation of oxides, which are catalyticallyineffective and block access to the catalyst.

According to the invention, the melting point of a metal based on noblemetal, in particular based on metals of the platinum group metals, suchas platinum, palladium, rhodium, ruthenium, and iridium, and, ifnecessary, additional alloy elements of up to 30% by weight in total ofthe metals nickel, cobalt, gold, rhenium, molybdenum and tungsten, isreduced substantially, in particular by at least 400° C., preferably byat least 500° C., by boron or phosphorus. For this purpose, eutecticcompositions are preferably produced with boron or phosphorus. Sucheutectic mixtures are generally situated between 1 to 5% by weight ofboron or phosphorus, based on the metal to be processed.

It is still feasible to carry out the present invention if the mixtureis not eutectic. However, the effect becomes unusable below 0.5% byweight of boron or phosphorus because the wide melt interval of thealloy leads to considerable demixing. If the proportion of boron orphosphorus is too high, boron or phosphorus is burnt unnecessarily, onthe one hand, leading to the further disadvantage, in addition to theincreased consumption, of the effort involved in removing this elementalso increasing unnecessarily. With a concentration above 10% by weightand with a rising boron or phosphorus content, the disadvantages in theform of the high consumption of boron or phosphorus, and the increasingeffort to remove them, must be considered in relation to the constantlydecreasing advantage of the reduction in the melting point even in thecase of compositions that have not reached a eutectic until then.

The reduction of the melting point according to the invention isparticularly marked in the case of platinum, iridium and their alloys.The use of boron to reduce the melting point is preferred since greatersafety measures have to be taken in general when using phosphorus.

According to the invention, fibers having a diameter of between 10 and200 μm, in particular between 50 and 100 μm, are obtainable. Drawingfibers from the melt, in particular by melt extraction, according to theinvention saves energy by reducing the melting temperature by boron orphosphorus and is gentler on the equipment, particularly in comparisonwith the effort involved without melting point reduction or incomparison with the effort involved in the production of wire sections.By reducing the melting point according to the invention, it is possibleto produce the fibers also by a wire casting process according to Germanpublished patent application DE 197 57 093 A1 or a melt spin processaccording to German published patent application DE 31 36 303 A1.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, apart from the simplified fiber productionby drawing from the melt, in particular by melt extraction, the sintertemperature and time necessary for the production of nettings ornon-wovens from the fibers is also reduced, without the catalytic andmechanical properties of the non-wovens being negatively affected, sinceboth boron and phosphorus are largely removed again before the use ofthe fibers, non-wovens or nettings as intended. The removal of boron andphosphorus takes place partly during drawing of the fibers and duringsintering of the fibers to form nettings or non-wovens. An excessivelyhigh residual content of boron or phosphorus negatively affects thedesired properties for application as catalyst.

It has proved to be particularly successful for removing the boron orphosphorus to heat the product, after sintering of the fibers to formnettings or non-wovens, for a few minutes to white heat. During such atreatment step, the strength of the material increases, while the boronor phosphorus content is reduced. It is assumed that boron or phosphorusremains above the usual contaminants by these elements, in particular atbetween 1 and 500 ppm. When boron is reduced to 1 to 20% by weight orphosphorus to 5 to 20% by weight, in particular, no negative effects ofthese elements on the catalytic action and mechanical stability of thefibers, nettings or non-wovens produced according to the invention needbe expected.

When using non-noble alloy elements or metals of the platinum groupsensitive to oxidation, such as iridium and ruthenium, the oxidation ofthe boron and/or phosphorus does not take place in an air atmosphere butunder less severe conditions, such as in an oxidizing gas mixturecontaining H₂/H₂O. This gentle method is advantageous in particular forpre-sintering, if metals sensitive to oxidation are contained in thealloy.

In the following, the present invention is illustrated by way ofexamples. By adding approx. 2% by weight of boron to platinum, themelting point can be reduced from 1770° C. to 790° C. This facilitatesthe production of fibers from Pt alloys, such as PtRh5, by the meltextraction process. The melting point of Pd is reduced in a similar wayfrom 1555° C. to 1065° C. by additionally alloying with approx. 3% byweight of B.

The PtRh5 fibers alloyed with boron are pre-sintered at a temperature(e.g. of 750° C.) just below the eutectic. The boron is oxidized duringpre-sintering, and the boron oxide formed is largely liquefied orevaporated. Also, first sintered compounds are formed between thefibers. By a brief sintering treatment (lasting a few minutes) at atemperature in the region of 1200° C.-1400° C. (e.g. using a flame), thenon-woven material is firmly sintered together. Traces of boron oxide,which may remain, can be eliminated by rinsing in warm water.

Non-wovens of palladium alloys can be produced in an analogous manner.Since, however, the eutectic of the Pd—B system of 1065° C. issubstantially higher and/or the melting point of Pd of 1555° C. issubstantially lower than that of platinum, greater care is required whenadjusting the temperature for pre-sintering and sintering. However, thetemperatures which are suitable can be easily determined by simple agehardening tests and subsequent metallographic examination of themicrograph.

Practical Example

5 kg of a PtRh5 alloy pre-melted in a conventional manner were rolled,after casting, to form a rod having a diameter of 10 mm and cut intolengths of approximately 30 mm. The sections were subsequently heatedslowly by induction in a zirconium oxide crucible, while being blanketedwith argon, 2.1% by weight of boron granules—corresponding to the Pt—Beutectic—being added to the melt. After brief melting, a temperatureincrease of the melt to above 1000° C. was carefully avoided, in orderto reduce the risk of a reaction between the boron and zirconium oxidein the crucible to a minimum. The alloy thus produced was cast in copperingot molds to form ingots of approximately 20 mm×20 mm×120 mm.

The prepared ingots of B-containing PtRh5 alloy were melted in azirconium oxide crucible in a melt extraction device (comparedescription in DE 199 45 742 C1, column 2, from line 40 onwards) underan argon blanketing atmosphere, while the temperature of the melt bathwas kept in the region of 820-860° C. Comparative tests for theproduction of fibers from the alloy PtRh5 without boron failed as aresult of the high melting temperature of the alloy (liquidustemperature approx. 1820° C.) and the damage thus caused to the meltcrucible. The extraction process was adjusted by preliminary tests insuch a way that 2.6 kg of fibers having a diameter of 50-60 μm and anaverage length of 5 mm were produced from the PtRh5-B alloy.

In line with the process described in DE 197 12 625 A1, the fibers werescattered onto a level substrate of aluminium oxide having thedimensions 400 mm×800 mm until the mass per unit area of the fibersreached 1500 g/m².

The non-woven material thus laid was introduced on the substrate into achamber furnace under an air atmosphere, heated to 750° C. and kept atthis temperature for 5 hours. The boron diffused from the fibers,oxidized at the surface of the PtRh alloy and formed boron oxide, whichis liquid at this temperature (melting point 450° C.). The substratewith the pre-sintered non-woven material was removed from the oven andcooled to room temperature. After cooling, it was possible to dissolvethe boron oxide in warm water, as a result of which the non-wovenmaterial detached itself from the substrate.

As a result of pre-sintering, an adequate mechanical strength forhanging up the non-woven material with platinum wire had already beenachieved. At this stage, however, the non-woven material was still veryfragile and had to be handled with great care. The non-woven materialwas heated with a hydrogen-oxygen flame to incandescence (roughlyapproximately 1200° C.) and kept at this temperature for 2-3 minutes.Following this treatment, the strength had increased considerably. Thestrength was determined by firmly stretching a circular non-woven blank(diameter 95 mm) around the circumference and pressing a steelhemisphere (diameter 40 mm) into it until the non-woven material broke.The maximum force reached was measured as being 95 N. In comparison, twonon-wovens were tested which had been produced by laying and sinteringfrom drawn PtRh5 wires and which also had a mass per unit area of 1500g/mm². The first non-woven material was sintered for 10 min at 1640° C.in the oven and had a compression strength of 10 N. The second non-wovenmaterial was sintered for 12 hr at 1350° C. and had a compressionstrength of 85 N. The non-woven material according to the inventioncould be handled without problems. The boron content of the PtRh5 alloyhad decreased to <0.001%. The non-woven material had a porosity of 91%.

Four circular blanks having a mass per unit area of 1500 g/m² anddiameters of 62 mm each were cut from the non-woven material. Thecircular blanks were tested in a test reactor for the oxidation ofammonia to nitrogen oxide under a load of 22.4 tonnes nitrogen persquare meter and day (t N/m²/d), a pressure of 3.5 bar and a temperatureof 860° C. In comparison, ten standard catalyst nettings, which had beenproduced by weaving 76 μm wires of PtRh5 and had a mass per unit area of600 g/m² each, were tested in a parallel reactor.

The pressure losses from the non-wovens were higher by a factor of 1.4than in the case of the nettings, the concentration of laughing gas inthe product gas was comparable in the case of the non-wovens and thenettings. The oxidation efficiency of the non-wovens was 96.2% under theconditions tested and remained at this level during the entire testperiod of 126.0 hours. The oxidation efficiency of the netting was 96.0%at the beginning of the test and fell to 95.5% in the course of the testperiod.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method for production of nitrogen oxide or hydrocyanic acid, themethod comprising using a non-woven material or netting as a catalyst,wherein the non-woven material or netting comprises metal fibers basedon one or more elements selected from the group consisting of platinum,palladium, rhodium, ruthenium, and iridium with 0 to 30% by weight ofone or more additional alloy elements selected from the group consistingof nickel, cobalt, gold, rhenium, molybdenum, and tungsten, and whereinthe metal fibers contain 1 to 500 ppm by weight of boron or phosphorus.